The invention relates to a projection display device comprising an illumination system having a light source and an optical guiding means for providing an illumination beam, color filter means comprising a color filter wheel for scanning the display panel with different colored portions of the illumination beam, and an image display system having an image display panel for modulating the colored portions of the illumination beam with image information and projecting an image on a screen.
The invention also relates to a color filter wheel for use in such a projection display device.
Projection display devices can be used in both rear and front image projection systems. In a rear projection system, the projection display device projects an image representing television or datagraphic information on the rear side of a diffusing transparent screen, which front side is directed to a viewing audience. In a front projection system, the projection display device projects an image representing television or datagraphic information on the front side of a reflecting screen, which front side is directed to a viewing audience.
Such a projection display device is known from U.S. Pat. No. 6,005,722. The known projection system comprises an illumination system and an image display system having an image display panel for modulating an illumination beam to be supplied by the illumination system with image information. The illumination system comprises a light source and an integrator system for forming an illumination beam. A color wheel is present between the light source and the integrator system. The color wheel includes red, green and blue filters. The color filters occupy angular segments of the color wheel. The color wheel filters the white light from the illumination system to produce a colored illumination beam that changes colors from red to green, from green to blue and from blue to red and so on in a rate proportional to the angular speed of the wheel. The display panel modulates the colored illumination beam in accordance with corresponding image information of the color of the illumination beam incident on the display panel. In order to improve the light efficiency, a reflective polarizer may be present between the color filter wheel and the display system. The reflective polarizer transmits a portion of the illumination beam having a polarization in a first direction and reflects a portion of the illumination beam having a second polarization in a second direction, perpendicular to the first direction. Furthermore, light recycling means may be present in the projection display device to reuse the reflected light from the reflective polarizer. A dichroic filter can also be used in the color filter wheel for reflecting a portion of the illumination beam having an undesired color.
The known projection display device has the drawback that the known color wheel with dichroic filters is difficult to manufacture.
It is an object of the invention to provide a projection display device and a color filter wheel which can be easily manufactured. This object can be achieved by the projection display device in accordance with the invention, and is characterized in that the color filter wheel comprises a cholesteric layer for reflecting a portion of the illumination beam not having a predetermined color and having a circular polarization corresponding to a first direction. The application of dichroic type color filters comprising cholesteric layers simplifies the manufacture of color wheels with, for example, spiral-shaped color filter portions. The invention is based on the recognition that the spectral properties of a cholesteric filter are dependent on the illumination conditions at different locations on the substrate during polymerization. By illuminating a liquid crystal on a transparent substrate with different light conditions in predetermined filter portions, and photopolymerisation of the different filter portions in a predetermined order and under predetermined light conditions, a dichroic filter pattern can be created. The direction of rotation of a helix of the cholesteric molecules forming the cholesteric layer and the pitch of the molecular helix of the cholesteric material determines a color or passband and a circular polarization direction of a transmitted portion of the illumination beam. Cholesteric color filters are known per se from the published international application WO 00/34808. In this application, a method of manufacturing a cholesteric mosaic color filter for direct-view liquid crystal displays is described. However, the filter portions in this mosaic color filter have dimensions of the order of several 100 micrometers.
A particularly advantageous embodiment of the projection device according to the invention is characterized in that the projection display device further comprises a wideband cholesteric layer positioned between the cholesteric layer and the display panel for reflecting of a portion of the incident illumination beam having a circular polarization corresponding to a second direction, opposite to the first direction. With this arrangement of color filter and polarizer, only the portion of the illumination beam having the predetermined color and predetermined circular polarization is transmitted to the display panel.
A further embodiment of the projection display device according to the invention is characterized in that the wideband cholesteric layer is provided on the color filter wheel. This arrangement allows simple manufacture of all cholesteric layers on a single substrate.
A further embodiment of the projection display device according to the invention is characterized in that the projection display device comprises a quarter-wave plate positioned between the color filter means and the display panel for converting the circularly polarized transmitted illumination beam into a linearly polarized illumination beam.
Cholesteric color filters transmit only a portion of the illumination beam having a circular polarization. Modulation of the illumination beam by a liquid crystal display may require an illumination beam having a linear polarization. Furthermore, the projection display device may be provided with a transmissive liquid crystal display panel or a reflective liquid crystal display panel.
Another embodiment of the projection display device according to the invention is characterized in that the color filter wheel is provided with a further cholesteric layer positioned between the cholesteric layer and the display panel on the color filter wheel, the rotation of the helix of the cholesteric molecules forming the further cholesteric layer, corresponding to a second direction opposite to the first direction for reflecting a portion of the incident illumination beam not having a predetermined color and having a second circular polarization corresponding to the second direction. This arrangement of cholesteric layers transmits only a portion of the illumination beam having a predetermined color.
A further embodiment of the display device according to the invention is characterized in that the image display panel comprises a digital micro-mirrored display panel.